1. Field of the Invention
The invention relates to a method and portable apparatus for soldering leadless semiconductor modules to or desoldering them from leaded terminal pads on a printed wiring board.
2. Description of the Prior Art and Objects
The development of leadless semiconductor modules or chip carriers, as they are often referred to has necessitated new techniques for attaching them to or removing them from printed wiring or circuit boards both during initial assembly of a plurality of carriers on a board or for the purpose of repair when only a single carrier need be removed and replaced.
The problem arises from the fact that the new carriers are leadless in that they have only contact pads on the bottom side of the carrier which must mount to tin-lead pads on the printed wiring board in contrast to earlier carriers that had depending metal legs which were inserted into and soldered to plated through-holes in the printed wiring board. The application of heat required to melt the solder during removal and replacement must be done in a way so as not to damage either the replacement carrier itself, carriers mounted adjacent thereto or damage the pads on the printed wiring board.
Known devices use either heated air or a tool having an electrically heated appendage(s) to melt the solder on the board pad to effect removal or attachment of the carrier pads thereto. Those devices that use heated air have the drawback that the air is often uncontrollably hot and directed at the whole carrier instead of only the pad portions resulting in damage to the replacement carrier as well as to neighboring carriers and the printed wiring board itself. Those devices that consist of a tool having a heated appendage which fits around the perimeter of the carrier have the drawback that it is not usually possible to determine when the solder between each pad has melted which results in pads being pulled off and burning of the printed wiring board. Further, the heated appendages may conduct electrical energy to the carrier in the form of an electrostatic discharge thus damaging it internally.
It is therefore the primary object of the present invention to provide a superior method and apparatus for effectively removing and replacing leadless semiconductor modules or chip carriers.
It is another object of the invention to provide a device capable of transmitting heat directly to the working area of one carrier to prevent damage to surrounding carriers caused by large temperature rises.
It is a further object of the present invention to provide a device and lift-off method which is not destructive to the carrier and which protects the printed wiring board from damage.
It is a still further object of the invention to provide a device of the subject type which is compact, easily manipulated and which is relatively inexpensive to manufacture, use and maintain.
Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and accompanying drawing which are merely illustrative of the present invention: